Thermal sensors operate by absorbing radiation, such as infrared energy emitted by a hot object, and converting that heat into an electric signal. These sensors are also known as bolometers. Thermal sensors can be used as single pixel detectors or may be built as a linear or rectangular array for radiation imaging. Thermal imaging arrays can detect hot objects with high pixel resolution and video frame rates. Thermal sensors such as those used to sense thermal infrared radiation for thermal imaging or thermometry typically fall into three categories: thermistors, thermopiles, and pyroelectric.
Thermistors are a type of resistors whose resistance changes with temperature significantly more than standard resistors. Thermistors are generally used as temperature sensors, self-regulating heating elements, and for other applications. Thermistor sensors are mostly accurate but have a low sensitivity and require a current bias. Thermopiles are composed of thermocouples that convert thermal energy into electrical energy. The Seebeck effect causes a voltage proportional to the temperature difference to appear across thermopiles. The temperature of thermopile thermal sensors can be read by measuring this voltage. Thermopile sensors are generally accurate and more sensitive than thermistor sensors but often are very slow due to high series resistance which combines with gate input capacitance of a measurement circuit to form a low-pass filter. Pyroelectric sensors based on pyro-electricity concept. Certain pyroelectric minerals and crystals create electric charge when they are subject to temperature change. Pyroelectric sensors are fast and sensitive but generally inaccurate because the pyroelectric crystals exhibit leakage and hysteresis which corrupts their stored charge. Pyroelectric sensors cannot be used for thermal imaging due to image fade and ghosting effects.